Coating non-conductive substrate surfaces, such as for example plastic surfaces, with metal coatings is a practice that is variously used for affecting the surface properties of substrates.
Metallization of non-conductive surfaces to render then electrically conductive is widely used in the field of manufacturing integrated circuits, printed circuits, or other electric or electronic components. Furthermore, surfaces are coated for decorative reasons with metal layers having corresponding optical or haptical characteristics.
Principally it is possible to deposit metal layers galvanically or autocatalytically on substrate surfaces. Before galvanic deposition, the plastic surface to be metallized is made electrically conductive by appropriate pre-treatments. Such a treatment that provides the surface with conductivity is unnecessary for the autocatalytic deposition of metal layers.
Before galvanic or autocatalytic metallization, the non-conductive surface may be roughened to improve adhesiveness of the metal layer on the substrate surface. This can be realized by a mechanical treatment or by an appropriate chemical treatment, for example with swelling agents or etchants.
For this, different methods are known from the state of the art. Thus, German patent specification DE 101 24 631 (corresponding U.S. Pat. No. 7,025,867) discloses treating a plastic surface with an acid permanganate solution prior to direct electrolytic metallization of electrically non-conductive substrate surfaces.
German patent DE 197 40 431 C1 discloses an etching step in the metallization of an electrically non-conductive surface area, in which the surface is treated in an acidic etchant containing hydrogen peroxide. Beside hydrogen peroxide, the etchant can also comprise an acid such as phosphoric acid or also organic compounds such as propan-2-ol or p-phenolsulphonic acid.
German publication DE 195 10 855 discloses a pre-treatment of substrate surfaces with a chromic acid containing sulfuric acid etchant for a method of selective or partial electrolytic metallization of substrates on non-conductive materials. Alternatively, an alkaline permanganate solution is disclosed as etchant.
WO 99/13696 discloses a pre-treatment of surfaces to be metallized by means of a pickling or etching composition comprising hydrogen peroxide for a metallization method of a substrate comprising electrically non-conductive surface areas. The disclosed etching compositions can furthermore contain phosphoric acid, methane sulfonic acid or ethanoic acid, wherein the hydrogen ionic concentration is limited to about 0.5 mol/kg solution.
German patent application DE 100 54 544 (corresponding U.S. Pat. No. 6,902,765) also discloses a pre-treating etching method of substrate surfaces by means of a solution containing chromate ions for a method of chemical metallization of surfaces, in particular surfaces of acrylonitrile/butadiene/styrene copolymers (ABS copolymers) and of a mixture of these copolymers. The solutions comprise chromium trioxide and concentrated sulphuric acid.
In all etching methods, it is important to solubilize the substrate surface in order to form the required adhesive surface for the metal layer to be deposited.
Substrates to be coated can be for example plastics, such as polyester, polyether, polyimides, polyurethanes, polyamides, epoxy resins, polysulfones, polyethersulfones, polyetherimides etc.
However, in particular for substrates based on polyamide, the methods known from the state of the art for preparing surfaces, such as the etching with chromic-sulfuric acid mixture, alkaline solutions, or acids, can cause problems since these pre-treatment methods lead to irreversible deterioration of the polyamide surface.
An approach for solving these problems is the surface etching method disclosed in the magazine metalloberfläche volume 59 (2005) no. 4 on pages 55 and following, which uses a reduced chromium trioxide concentration. Hitherto, about 400 g/L chromium trioxide have been used in the state of the art, but the here disclosed method uses a composition comprising about 80 g/L chromium trioxide.
But the use of chromium trioxide is difficult because of the environmental problems caused by chromates.